Process for producing high density uranium carbide-plutonium carbide pellets



United States Patent This application is a continuation-in-part application of our co-pending application Ser. No. 268,555, filed Mar.

28, 1963, now abandoned.

The present invention relates to fuels for nuclear reactors and has particular reference to fuels suitable for use in fast reactors.

It will be known that under the conditions contemplated for fast reactors it is necessary to use an enriched fuel and very conveniently this enriching may be achived by incorporating a proportion of plutonium, e.g., 15 atomic percent, to replace the uranium which is the basic fuel. Moreover, at the reactor fuel temperatures contemplated, which are in the region of 800 C. at the surface of the fuel or fuel element can, it is clearly impracticable to use uranium metal as the fuel by reason of the phase changes and low melting point of this metal.

Several alternative proposals have already been put forward and one of these proposals is the use of a fuel comprising uranium carbide. At the present time it appears that this carbide fuel will prove most satisfactory as it has a considerably higher thermal conductivity than uranium dioxide, although it is known that under certain conditions it reacts rapidly with air or oxygen.

It is the object of the present invention to provide a process for producing an enriched uranium carbide fuel.

According to the present invention there is provided a process for producing a high density, enriched uranium carbide fuel material comprising the steps of providing an admixture of uranium and plutonium carbides, said admixture having a particle size substantially in the range 2 to 4 microns, a carbon content in the range 48 to 53 atomic percent and an oxygen content below 1 percent by weight, compacting said powder into pellets and sintering the pellets in an inert atmosphere of high purity inert gas.

Using the method of the present invention densities in excess of 95% theoretical density may be obtained with a sintering temperature as low as about 1550 C.

Previous attempts to produce high density sintered bodies have required temperatures of about 2000 C. or a sintering aid e.g., nickel, to obtain densities of about 90% theoretical since by sintering the mixed carbides in vacuo at 1650 C. densities of ony 55% of theoretical have been obtained.

We believe that the difficulties experienced by previous workers in obtaining satisfactory sintered products were mainly due to the fact that excessive oxygen was introduced into the powders either before or during sintering and it appears that the. presence of large quantities of oxygen in the powder results in poor sintering. Thus, in the present invention, in order to obtain satisfactory sintering, it is necessary, inter alia, that the oxygen content of the starting powder should be low and that the pellets obtained by compacting this powder should be sintered in an atmosphere which is inert and of high purity. The inert gas used for the sintering atmosphere is preferably argon having an oxygen content of less than 10 parts per million and a water vapour content of less than 10 parts per million. This atmosphere is conveniently obtained by passing high purity argon through a molecular sieve and over hot uranium.

The very fine powders required as the starting material for the process of the present invention rapidly absorb oxygen and water vapour from the atmosphere and we have found that when the powders were prepared in an argon atmosphere containing less than parts per million of oxygen and 100 parts per million of water vapour, they showed a weight gain of approximately /2 percent per hour. Thus, even in an atmosphere containing quite small amounts of oxygen and water vapour, oxidation of the powders occurs at quite a fast rate. Therefore, if such an atmosphere is used in preparing the powders for use according to the present invention, it is necessary to effect all operations extremely rapidly in order to minimise the oxidation of the powder. a

It will be appreciated that in the present process, care is taken to ensure purity of the sintering atmosphere in contact with the mixed carbides, and this requires not only a supply of high purity gas but also precautions to prevent even small leaks which would result in oxygen and water vapour contamination of the gas supply. Previous workers do not appear to have taken such precautions and we believe that the difficulties they have encountered in attempting to obtain a high density product are due, at least partly, to their apparent failure to take the necessary precautions to maintain a sufiiciently pure atmosphere.

The mixed carbide material which is used in the present process may be prepared by any suitable known method, for example, are melting the metal with carbon, or reacting the oxide with carbon. In either case the carbides may be prepared together or separately and subsequently mixed together into the appropriate amounts. During both the preparation of the carbides and the subsequent handling to form the fine mixed carbide powder which is the starting point of the present process, the oxygen content of the material must be maintained below the specified maximum permissible figure of 1% by weight. It will however be realised that although the handling conditions during the preparation of the fine powdered starting material of considerable importance in ensuring the purity of the starting material, this does not form part of the process of the present invention but is merely a preliminary step to the present process. The important factors in the present invention however are the purity of the sintering atmosphere and the nature of the mixed carbide powder starting material and the method of preparation of the starting powder is important only in so far as it ensures that the powder satisfies the requirements specified. It will be apparent that unless proper precautions are' taken during preparation of the powder, it will not conform to the minimum requirements.

In the phase diagram of the three-component system uranium-plutonium-carbon, we have discovered that there are conjugation lines between uranium monocarbide and plutonium monocarbide, between uranium sesquicarbide and plutonium sesquicarbide and between uranium monocarbide and plutonium sesquicarbide. The preferred degree of enrichment that is required for the fast reactors under study corresponds to 15 atomic percent of plutonium and with this proportion of plutonium, the proportion of carbon from 50 to 53 atomic percent lies in that portion of the phase diagram which corresponds, in the product, to a coherent matrix of uranium carbide, containing possibly a small proportion of plutonium carbide in solid solution and a disperse phase of plutonium sesquicarbide. In theory the products having less than 50 atomic percent of carbon should show a disperse phase of uranium and plutonium metal but in point of fact we have not found such a disperse phase and we have deduced that a small percentage of oxygen is present in solution in the monocarbide. Thus, carbon contents below 50 atomic percent are permissible and the minimum carbon content is 48 atomic percent. The small amount of oxygen has not been positively identified but on the other hand the presence of oxygen, which has been established by analysis, has not been detected by any form of microscopic or X-ray examination of the finished product, and this supports the view that such oxygen is present in solid solution in the monocarbide phase.

So far as the present experiments have been carried, we believe that of the three components present, namely uranium monocarbide, plutonium monocarbide and plutonium sesquicarbide, the highest thermal conductivity is possessed by the uranium monocarbide and thus it is advantageous to provide a product in which this phase is present in the form of a coherent matrix. Consequently, compositions are preferred containing approximately 52 atomic percent carbon so that the majority of the plutonium is present in the form of the sequicarbide and little is present in solid solution in the uranium monocarbide which therefore has its highest conductivity value.

Example In a preferred arrangement in accordance with the invention, a sample of mixed carbide which had been prepared by arc melting the metals with carbon in the requisite proportion was ground for a long period of time (about 20 hrs.) in a ball mill and the product thus obtained was sieved to retain the fraction smaller than 4 microns, this fraction being predominately larger than 2 microns. The are melting to form the mixed carbides was conducted in such a way that the amount of impurities were kept to a minimum and in particular graphite electrodes were used since with tungsten electrodes the small amount of tungsten introduced during the arc melting appears to prevent satisfactory sintering. The milling was eifected in such a way that the oxygen content of final powder was less than 1%.

A 25 gm. portion of the mixed monocarbide powder thus prepared was then divided into five lots and each of these was formed into a pellet by being compacted under a pressure of 40 to 100 tons per square inch. Naphthalene, in an amount of /2 to 1 percent by weight, could be used as a binder if desired, or alternatively, a die wall lubricant might be preferred.

The compacts thus obtained had a green density of between 8 and 9 gms./cc. and were then sintered in an argon atmosphere of high purity at 15 C. for approximately four hours whilst being supported on molybdenum boats. The sintered product had a silvery appearance and a bulk density of 13.2 gms./ cc. Specific gravity measurements gave a value in the region of 13.2 gms/cc., thus showing that the open porosity of the product was very small. It should be noted that the theoretical density of the material was 13.6 gms/cc.

We claim:

1. A process for producing a high density enriched uranium carbide fuel body comprising the steps of providing a powder having a particle size distribution of essentially all particles in the range 2 to 4 microns, said powder comprising an admixture of uranium carbide and plutonium carbide having a carbon content of 48-53 atomic percent and containing less than 1% by weight of oxygen, compacting said powder into pellets and sintering said pellets in an inert atmosphere of high purity inert gas.

2. The process of claim 1 wherein the high purity inert gas is argon containing less than 10 parts per million of oxygen and 10 parts per million of water vapour.

3. The process of claim 2 wherein the sintering is effected at approximately 15 50 C.

4. The process of claim 1 wherein the plutonium carbide content corresponds to 15 atomic percent plutonium with respect to the total number of metal atoms.

5. The process of claim 4 wherein the carbon content is 53 atomic percent.

References Cited UNITED STATES PATENTS 3,082,163 3/1963 Ogard et al. 252-3011 3,162,528 12/1964 Williams et a1 -214 3,166,515 1/1965 Sowden 252301.1 3,236,922 2/1966 Isaacs et a1 252301.1

CARL D. QUARFORTH, Primary Examiner.

BENJAMIN R. PADGETT, Examiner.

S. I. LECHERT, JR., Assistant Examiner. 

1. A PROCESS FOR PRODUCING A HIGH DENSITY ENRICHED URANIUM CARBIDE FUEL BODY COMPRISING THE STEPS OF PROVIDING A POWDER HAVING A PARTICLE SIZE DISTRIBUTION OF ESSENTIALLY ALL PARTICLES IN THE RANGE 2 TO 4 MICRONS, SAID POWDER COMPRISING AN ADMIXTURE OF URANIUM CARBIDE AND PLUTONIUM CARBIDE HAVING A CARBON CONTENT OF 48-53 ATOMIC PERCENT AND CONTAINING LESS THAN 1% BY WEIGHT OF OXYGEN, COMPACTING SAID POWDER INTO PELLETS AND SINTERING SAID PELLETS IN AN INERT ATMOSPHERE OF HIGH PURITY INERT GAS. 